XVI INTRODUCTION. 



respecting the rise and motion of the sap in plants, 

 the perspiration, or evaporation, which is constantly 

 going on to a greater or less extent, from the surface 

 of the leaves, and the effects of various substances 

 on plants; his chemical speculations, however, are 

 for the most part, of little value, though he was often 

 apparently on the point of making important disco- 

 veries. The discovery of carbonic acid gas, or fixed 

 air, by Dr. Black, and the beautiful experiments of 

 Priestley, opened a new field of inquiry and research : 

 he observed that plants possessed the property of 

 purifying the air; in fact, that they were able to 

 decompose the carbonic acid gas which it always con- 

 tains in small quantity; appropriating the carbon, 

 and restoring back to the atmosphere the oxygen, or 

 vital air, so necessary to the processes of respiration 

 and combustion. The knowledge of this great fact, 

 necessarily led to many minor discoveries respecting 

 the growth of plants, and the sources of their food. 

 After this period. Organic Chemistry began to attract 

 a large share of the attention of chemists, the com- 

 position of vegetable substances was carefully inves- 

 tigated, new modes of analysis were discovered, and 

 an immense mass of curious and useful facts was col- 

 lected. A great number of chemists occupied them- 

 selves with researches in Vegetable Chemistry, but 



